In video signal processing and acquisition systems, a so-called "frame grabber" is typically used to acquire a copy of an image characterized by video signals generated by an image scanner for delivery to a display device, such as a raster scan display. Such video signals typically include signals defining text. To acquire a copy of the video signals, the frame grabber, using a sample clock signal that is phase synchronized with a horizontal sync signal that is outputted by the image scanner, digitally samples each of the video signals. In this way, the frame grabber generates pixel elements (pixels) each having a so-called grey scale value indicative of the level of a respective one of the sampled video signals.
In a monochrome system, video signals representing text typically define black or white, whereas video signals representing an image other than text may define black, white or different shades of grey. To ensure that the values of pixels obtained as a result of sampling video text signals define text, rather than some other image, then, the phase of the sampling clock signal needs to be synchronized with the steady state period of such video text signals. However, such synchronization is difficult to achieve simply and inexpensively using only the horizontal sync signal. In fact, when the phase of the sample clock signal is synchronized solely with horizontal sync signal, then it is likely that such video text signals will be sampled during their transition times, thereby resulting in pixel values defining respective shades of gray.